1. Field of the Invention
The present invention relates to an image forming technique.
2. Description of the Related Art
Generally in image forming apparatuses such as copying machines and printers of an electrophotographic method and electrostatic printing method, it is necessary to supply a developing material at a proper timing in order to maintain high-quality image output. In terms of color reproduction, color image forming apparatuses which form full-color and multi-color images often use a developing material (two-component developing material) containing two components: a toner particle serving as a coloring material, and a magnetic powder called a carrier particle. In an image forming apparatus using the two-component developing material, the toner density (i.e., the ratio of the toner particle weight to the total weight of the carrier particle and toner particle) of the two-component developing material is very important to stabilize the image quality. The toner particle in the two-component developing material is consumed in development to change the toner density. For this reason, it is necessary to detect the toner density of the developing material in a development device at a proper timing by using a developing material density control device, supply toner in accordance with the change of the toner density, control the toner density to be always constant, and maintain the image quality.
In general, the developing material density control device includes a developing material density detector which detects the toner density of the two-component developing material in the development device, and a controller which controls to supply toner to the development device in accordance with a signal representing the detected toner density. As the arrangement of the developing material density detector, a light quantity detection type toner density detection sensor has conventionally been proposed. This sensor irradiates a two-component developing material with near infrared light on the development sleeve of a developing material carrier, and receives reflected light to detect the toner density.
FIGS. 1A and 1B are a sectional view and plan view, respectively, showing the schematic structure of a development device in a conventional image forming apparatus, and an example of a conventional optical reflected light quantity detection type toner density detection sensor used in the development device. FIG. 1B is a plan view of a developing device 101 shown in FIG. 1A when viewed from above. The developing device 101 is arranged to face an image carrier 102 such as a photosensitive member or dielectric. The interior of the developing device 101 is divided into a development chamber (first chamber) 104 and a stirring chamber (second chamber) 105 by a partition 103 extending in the vertical direction. The development chamber 104 and stirring chamber 105 store a two-component developing material containing a nonmagnetic toner and magnetic carrier.
The development chamber 104 and stirring chamber 105 incorporate screw type first and second developing material stirring/conveyance mechanisms 106 and 107, respectively. The first and second developing material stirring/conveyance mechanisms 106 and 107 function as a supply unit for supplying a stirred developing material. The first stirring/conveyance mechanism 106 stirs and conveys a developing material in the development chamber 104. The second stirring/conveyance mechanism 107 stirs and conveys toner supplied from a toner supply tank (not shown) via a toner supply port 108 arranged upstream of the second stirring/conveyance mechanism 107. At this time, the developing material in the stirring chamber 105 is also stirred and conveyed to make the toner density uniform. As is apparent from FIGS. 1A and 1B, developing material paths are formed on the two ends of the partition 103 to allow the development chamber 104 and stirring chamber 105 to communicate with each other. By a conveyance force in directions indicated by arrows A and B in the first and second stirring/conveyance mechanisms 106 and 107, a developing material in the development chamber 104 in which toner is consumed by development to decrease the toner density moves into the stirring chamber 105 via one path. A developing material which recovers the toner density in the stirring chamber 105 moves into the development chamber 104 via the other path.
The development chamber 104 has an opening at a position corresponding to the development area facing the image carrier 102. A development sleeve 109 serving as a developing material carrier is rotatably arranged to be partially exposed from the opening. The developing sleeve 109 is formed from a nonmagnetic material, and rotates in a direction indicated by an arrow in FIG. 1A in the development operation. A magnet serving as a magnetic field generator is fixed inside the developing sleeve 109. The developing sleeve 109 carries and conveys a layer of the two-component developing material, the thickness of which is regulated by a blade. In the development area facing the image carrier 102, the developing material is applied to a latent image on the image carrier 102, developing the latent image. To increase the development efficiency, that is, the ratio of toner applied to a latent image, a development bias voltage obtained by superposing DC and AC voltages is applied from a power supply 110 to the development sleeve 109.
An optical reflected light quantity detection type toner density detection sensor 111 is formed from an LED and photodiode. The LED emits light to the two-component developing material on the development sleeve 109. The photodiode detects reflected light whose quantity changes in accordance with a change of the toner amount, and converts the reflected light into an electrical signal. The difference between the signal value of the electric signal and a reference value is calculated. The developing material is supplied to the stirring chamber 105 via the toner supply port 108 by an amount determined in accordance with the difference. In order to correct changes of the output values of a light-emitting element and light-receiving element upon a change of the temperature, it is often to use a bidirectional emission LED and two photodiodes, receive direct light from the LED by the second photodiode, and set the detection output as a reference signal.
FIG. 2 is a flowchart showing a toner supply operation. When the printing operation starts and the development sleeve 109 and first and second developing material stirring/conveyance mechanisms 106 and 107 start rotating, the toner density detection sensor 111 detects the toner density of the developing material on the development sleeve 109 (S201). If necessary, the detection output is amplified. Then, the output is converted into a digital signal by an analog-to-digital converter (A/D converter), and the digital signal is supplied to an arithmetic circuit (S202). The arithmetic circuit compares the input signal with a reference signal to calculate the difference between them. The arithmetic circuit calculates the change amount of the toner density from the difference, and supplies a toner density change amount signal representing the change amount to a toner supply circuit (S203). The toner supply circuit converts the received toner density change amount signal into a toner supply amount (supply time) (S204). The second developing material stirring/conveyance mechanism 107 of the toner supply path is driven by a converted supply time to supply a predetermined amount of toner (S205).
As another method, an inductance detection type toner density detection sensor is used to detect the inductance of the developing material in the development device and obtain the toner density. As still another method, the toner density detection sensor detects a toner density from a reference image (toner image) patch formed on the image carrier. In all the three methods, the change amount of the toner density of a two-component developing material is obtained by the toner density detection sensor, and converted into a toner supply amount (supply time) to supply a predetermined amount of toner from the toner supply tank.
In addition to the above-described conventional methods, there is proposed a so-called video count type developing material density control device used particularly in a digital image forming apparatus (see, e.g., Japanese Patent Laid-Open No. 5-323791). According to the video count method, the output levels of all or some input image signals are converted for respective pixels to calculate the video count. The toner consumption amount is predicted from the video count to determine a toner supply amount so as to keep the toner density in the development unit constant.
However, in the conventional developing material density control based on a detected toner density, the toner supply amount is obtained from the toner density change amount of the developing material. The toner density of the developing material in the development chamber is fed back to supply toner. For example, when outputting an image which exhibits high density on the entire surface and consumes a large amount of toner, even the toner density which is uniform in the development unit before output abruptly decreases, making the output image unstable.
The video count type developing material density control can predict a toner consumption amount to a certain degree even for an output image for which the toner consumption amount greatly varies. However, the toner supply position is set upstream of the stirring chamber of the development device, as described above. It takes some time to move the developing material from the stirring chamber to the development chamber. Generally in the video count method, the toner supply amount is determined parallel to the rendering processing of an image signal to be printed, and toner is supplied at almost the same time as printing. Since neither movement nor diffusion of the developing material in the development unit is considered, the supply amount is not appropriate and the toner density becomes nonuniform.